The present invention relates to a method for disconnecting a battery in a hybrid electric vehicle, and further to a computer program and a computer program product for performing such a method.
A Parallel Hybrid Electrical Vehicle (Parallel HEV) configuration has an. engine, e.g. an Internal Combustion Engine (ICE), and an electric machine that together provide the necessary wheel, torque to drive the vehicle. Additionally, in the Parallel HEV configuration, the electric machine can be used as a generator to charge a battery pack from the power produced by the ICE. The is usually provided with a transmission between the ICE and drive wheels of the vehicle in order to be able to alter gear ratio between the ICE and the drive wheels and also in many cases between the electric machine and the drive wheels.
In a Parallel HEV comprising a hybrid electric powertrain with a combustion engine and an electric drive, the energy storage system (ESS) for the electric drive is essential. The function of the ESS is as an energy buffer so that an electric machine can be used both as a traction voltage alternator to charge the energy storage and as an electric motor to at least assist the combustion engine in the propulsion of the vehicle whereas the energy is taken from the ESS. The hybrid electric system operates at a traction voltage level, which can be typically 200-950V depending on the hybrid electric powertrain components and the system design.
The traction voltage in a Parallel HEV is not only used to power the vehicle through the electric machine, but is often also used to power electrical auxiliary units, some of which are essential for the vehicle's function.
Typical examples of essential electrical auxiliary units powered from the traction voltage are low voltage DC/DC converters used when the alternator is broken or when the capacity of the alternator is not enough, electric power steering that replaces a mechanically driven power steering driven by the powertrain, and electric air compressor that replaces a mechanically driven compressor driven by the powertrain. Without the function of these essential electric auxiliary units, the vehicle cannot be driven or has a very limited driving range.
A battery pack in an ESS is usually sensitive to temperature. The typical temperature operating range of a traction battery is between 0-50 degrees Celsius, where a good operation range can be limited to 10-40 degrees Celsius. A typical temperature operating range of a vehicle is −40-50 degrees Celsius. In the temperature range outside of the battery operating range, but within the vehicle operating range, the system cannot operate properly. This means that the powering of the essential auxiliary units may be interrupted and that the vehicle cannot be driven.
An ESS comprising a battery or a super capacitor is a complex electronic component that is added to the powertrain. The ESS is in itself a subsystem with a multitude of components: electronic, electric, chemical and/or mechanical components with functions for energy storage, monitoring, cooling and disconnection components. The added number of components and functions increase the risk of faults, which may have a negative impact on the vehicle operation since the ESS is essential for the operation of the vehicle.
In a Parallel HEV powertrain, the ESS is often designed with contactors to disconnect the ESS from the traction voltage DC-link in order to increase safety in a vehicle that is not operated. The traction voltage DC-link and its connected components will act as a large capacitor when an ESS, which is a voltage source, is connected to the traction voltage DC-link. A pre-charge circuit with a resistor can be used to obtain a controlled charging of the traction voltage DC-link before connecting the ESS. Otherwise, upon connection, a large current will flow through the contactors into the traction voltage DC-link which may damage the contactors. The pre-charge circuit is a part of the ESS, but it is only necessary when connecting the ESS to the traction voltage DC-link. After a pre-charge is completed, the pre-charge circuit will be passive until the next connection of the ESS to the traction voltage DC-link. A fault in the pre-charge circuit will lead to that the ESS cannot be connected to the traction voltage DC-link which in turn means that the vehicle cannot be operated.
A known solution to keep the battery in an operable state is to keep the battery within its operating temperature range by e.g. parking indoors in a temperate surrounding, or by connecting an external heater e.g. to the battery coolant system that continuously can maintain the battery temperature within battery temperature limits.
U.S. Pat. No. 8,120,200 discloses a hybrid vehicle provided with a failsafe backup mode in case of disconnection of the battery pack (20). If the battery pack is disconnected, e.g. due to an over temperature, the current supplied from the battery is reduced to zero and the electric machine (18) will start to function as a generator for supplying power to the loads (38) of the vehicle. The vehicle is provided with an auxiliary battery (40) which is also connected to the loads. In this way, the loads will still be supplied with current even if there is a short interruption of power when the battery is disconnected and the electric machine starts to deliver current.
It is desirable to provide an improved method for disconnecting the battery when needed.
It is desirable to provide an improved method for disconnecting a battery from a traction voltage DC-link in a hybrid vehicle.
In a method for controlling a hybrid vehicle electrical system comprising one or more electrical auxiliary units, a combustion engine driven electric machine operable as a generator for supplying power to said electrical auxiliary units via power electronics unit and via a traction voltage DC-link, a high voltage battery pack coupled to the traction voltage DC-link by a relay that can be opened to disconnect the battery pack from the high voltage DC-link under fault conditions, the method comprises the steps of.                driving the vehicle in a first driving mode where the battery pack is connected to said traction voltage DC-link and the battery pack is supplying the electrical auxiliary units with power,        upon receiving a first indication that said battery pack has to be disconnected, and upon receiving also a second indication that the vehicle is in a predetermined vehicle condition, then        shutting down the power electronics unit in order to cease power delivery from the electric machine to the traction voltage DC-link or to the electric machine from the traction voltage DC-link,        disconnecting the battery pack from the traction voltage DC-link by opening the relay, and then        activating the power electronics unit and controlling output voltage of the power electronics unit to a predetermined level, as long as the electric machine is rotating and supplying power through the power electronics unit to the traction voltage DC-link in order to resume operation of said electrical auxiliary units.        
By this first embodiment of the method, the method can control the electrical system of a hybrid vehicle, where the electrical system comprises one or more electrical auxiliary units and a battery, pack, such that the battery pack can be disconnected from the traction voltage DC-link of the vehicle in a controlled way, and that the operation of the auxiliary units can be continued after the battery pack has been disconnected. When the battery pack has been disconnected, the auxiliary units are powered by the electric machine acting as a generator. The electric machine is connected to the traction voltage DC-link through a power electronics unit adapted to control the output voltage of the power electronics unit to a predetermined level.
The electrical auxiliary units are preferably shut off before the battery pack is disconnected from the traction voltage DC-link, especially if the electrical auxiliary units are high power units. The advantage of shutting down high current loads before the battery pack is disconnected is to avoid surge currents during the disconnection.
The disconnection of the battery pack is initiated by an indication that the battery pack has to be disconnected. Such an indication may be e.g. a low external temperature, a too low or too high battery temperature, a faulty battery, a too high voltage of the battery or a faulty battery charger. Further, the disconnection of the battery is preferably also initiated by an indication of a vehicle condition. Such a vehicle condition may e.g. be that the combustion engine is running, and that the vehicle is either standing still or travelling forwards.
In a development of the inventive method, the predetermined vehicle condition is a condition where it has been secured that the auxiliary electrical units can be temporarily disconnected. A control unit of the vehicle can be programmed to monitor different auxiliary electrical units in the vehicle so that they are temporarily not used and therefore can be temporarily disconnected. If the current load of the electrical auxiliary units is low, e.g. below 20 A or 10 A, the surge current will be relatively low which means that the electrical auxiliary units do not have to be shut off before they are disconnected. Different sensors in the vehicle can be used in order to determine the prevailing vehicle condition and also coming vehicle conditions. The time span can be up to a few seconds, i.e. during the duration of the temporary disconnection of the different electrical auxiliary units.
If a driver during the temporary disconnection of the different electrical auxiliary units indicates that a functionality from one of said different electrical auxiliary units is needed, the control unit can be programmed to postpone the use of the electrical auxiliary unit until the power electronics unit has been activated again after the disconnection of the battery pack in order to control the output voltage of the power electronics unit to a predetermined level, in order to resume the operation of said electrical auxiliary units.
In a further development of the method, the electrical auxiliary units are at least one of a DC/DC converter from the DC-link voltage to 12/24V, electric power steering, an electric air compressor, an electric air condition or an electric compressor.
The advantage of the invention compared to prior art U.S. Pat. No. 8,120,200 is that no extra battery is needed in order to provide power during the time the main battery pack is disconnected. In this way a simpler and cheaper solution can be provided.